Microsoft has made available its RC (Release Candidate) for Windows 7 to the General public. Before you run off and download it to install, be aware of some potential problems. First make sure you have a PC it will run on. Also keep in mind, just because you have a PC that meet the requirements does not mean all your drivers will work. (i.e. sound, printers, network, video) Windows 7 has a virtual PC build in to allow it to run in XP mode. Again, not all PC can run windows 7 in XP mode. To run XP mode you will need 2 GB of RAM and a CPU the supports chip-level virtualization.
To check to see if your Intel CPU will support it go to: http://www.intel.com/support/processors/tools/piu/
For AMD CPU's go to: http://support.amd.com/us/Pages/dynamicDetails.aspx?ListID=c5cd2c08-1432-4756-aafa-4d9dc646342f&ItemID=172
For more information on Windows 7 go to: http://www.microsoft.com/windows/windows-7/download.aspx